In order to improve luminous efficiency of an organic EL device, a technology of adjusting the optical interference distance is widely used. Further, in a top-emitting organic EL device, as means to improve the luminous efficiency, to allow the emission spectrum to be narrow and sharp and to adjust the peak wavelength, a technology of adjusting emission spectrum by disposing an organic layer including an emitting layer between a metal electrode and a semi-transparent metal electrode to configure an optical resonator has been widely used. By this technology, the length of a resonator is adjusted by changing the distance between the metal electrode and the semi-transparent electrode. A method in which the film thickness of the organic layer is changed to adjust the optical interference has a defect that luminous efficiency is varied due to a change in carrier balance. Therefore, the thickness of an organic layer is restricted to one at which the carrier balance is maintained, and hence, the adjustment of the length of a resonator has its limit.
In a top-emitting organic EL device, as a technology of adjusting the optical interference distance without changing the carrier balance, a technology has been proposed in which a thin film structure of which the optical interference distance can be adjusted is provided on the upper semi-transparent metal electrode. In Patent Document 1, a resonator is provided on the upper electrode, and the length of the resonator is adjusted such that the optical path length of the device becomes integral multiple of the half-wave length, as in the case of a Fabry-Perot resonator, thereby to adjust the peak wavelength. Patent Document 2 discloses a technology in which an optical adjustment layer is formed on an upper electrode in a thickness of several nm to several hundreds nm, whereby the optical interference distance is adjusted.
As is apparent from the above-mentioned conventional technologies, a technology of providing on an upper electrode of a top-emitting organic EL device a thin film structure of several nm to several hundreds nm such that the optical path length becomes integral multiple of the half-wave length as in the case of a Fabry-Perot resonator is known in this technical field.
In a top-emitting organic EL device, a technology has been proposed in which a thin-film structure called a resonator, a protective film, a capping layer or an optical adjustment layer is added on an upper semitransparent electrode in order to improve luminous efficiency. In a blue emission range of wavelengths of from 400 nm to 500 nm, a technology of allowing the emission spectrum to be narrow and sharp and allowing the peak wavelength to be short while attaining highly efficient emission has not been proposed. In particular, the wavelength of light giving blue emission is short as compared with the wavelength of light of green or red, and the blue light has high energy. It is hence difficult to optimize the optical interference distance while keeping the carrier balance.
As the prior art invention in which emission properties have been improved by providing on the upper semi-transparent electrode a thin-film structure, the following can be given. In Patent Document 2, on the upper electrode of a top-emitting organic EL device, an organic capping layer having a reflective index of 1.7 or more and a film thickness of 600 Å is provided, whereby emission is strengthened by about 1.5 times in a red-emitting device and a green-emitting device.
Patent Document 3 discloses an invention in which suppression of natural light reflection and improvement in contrast by emission with high luminous efficiency due to the presence of an organic capping layer doped with a Nile Red dye on an upper electrode of a top-emitting organic EL electrode.